1. Field of the Invention
The present invention relates to polishing slurry, and more particularly to a chemical-mechanical polishing slurry that reduces wafer defects and its method of making.
2. Description of Related Art
In the manufacture of integrated circuits, the planarization of semiconductor wafers is becoming increasingly important as the number of layers used to form integrated circuits increases. For instance, metallization layers formed to provide interconnects between various devices may result in nonuniform surfaces. The surface nonuniformities may interfere with the optical resolution of subsequent lithographic steps, leading to difficulty with printing high resolution patterns. The surface nonuniformities may also interfere with step coverage of subsequently deposited metal layers and possibly cause open or shorted circuits.
Various techniques have been developed to planarize the top surface of a semiconductor wafer. One such approach involves polishing the wafer using a polishing slurry that includes abrasive particles mixed in a suspension agent. With this approach, the wafer is mounted in a wafer holder, a polishing pad has its polishing surface coated with the slurry, the pad and the wafer are rotated such that the wafer provides a planetary motion with respect to the pad, the polishing surface is pressed against an exposed surface of the wafer, and the slurry is used as a hydrodynamic layer between the polishing surface and the wafer. The polishing erodes the wafer surface, and the process continues until the wafer topography is largely flattened.
In chemical-mechanical polishing (CMP), the abrasive particles provide friction while oxidants and/or etchants cause a chemical reaction at the wafer surface. Additives can also be added to enhance the removal rate, uniformity, selectivity, etc., and dilution by deionized water is also practiced.
CMP is becoming a preferred method of planarizing tungsten interconnects, vias and contacts. Tungsten CMP slurries typically include abrasive particles such as alumina, a ferric salt oxidizer such as ferric nitrate, a suspension agent such as propylene glycol, and deionized water. With proper process parameters, CMP tungsten processing has shown significantly improved process windows and defect levels over standard tungsten dry etching. One significant advantage of CMP tungsten processing is that it has a highly selective polish rate for tungsten as compared to the dielectric. This selectivity allows for over-polishing while still achieving a flat tungsten stud. When overetching occurs using dry etching, the contact or via becomes further recessed, which creates a serious disadvantage since overetching is frequently required to remove defects.
The advantages of CMP, however, can be offset by the creation of significant defects during polishing, such as scratches. The prior art teaches that scratching can be controlled by the proper manufacturing, size control and filtering of the abrasive particles. The prior art also teaches that the proper mixing sequence of the abrasive particles with the suspension agent leads to lower defects.
Unfortunately, for various reasons, prior CMP slurries have not been as effective as needed. In particular, deep or wide scratch defects in the polished surface continue to cause problems. This may arise since conventional slurry filtering tends to remove only those particles that are significantly larger than most of the abrasive particles. Therefore, a need exists for an improved CMP slurry that reduces scratching defects.